Electrical machines are cooled to remove the heat generated within them. Heat is mostly generated in the active part of the electrical machine, the stator and the rotor, by magnetic and resistance losses.
Copper losses are created in the resistive conductors of electrical machine windings when part of the electric current transforms into heat. The variable magnetic flux causes iron losses in the iron parts of the motor and generator, including eddy-current losses and hysteresis losses.
In addition, mechanical losses occur in rotating electrical machines, such as bearings, or as turbulence on the rotor surface.
Heat losses thus created in machines must be conducted away to prevent the machines from heating until a balance is reached between the generated heat energy and the heat energy being conducted away from the machine.
Electrical machines are designed to comply with a particular insulation class. Winding insulation is selected to allow a reasonable lifetime in the chosen insulation class. If the electrical machine operates in an overheated condition due to overload or deterioration of the cooling system, the insulation lifetime will decrease rapidly.
Air is normally used as the coolant in small electrical machines of less than 1 . . . 2 MW. In larger electrical machines, liquid coolant is used; the most common liquid coolant is water. In many arrangements, the actual coolant is air that is cooled using water.
In liquid-cooled electrical machines, the cooling agent does not come into direct contact with the cooled components. The cooling agent is led into the electrical machine in closed pipes or channels, and removed from the machine when warmed up. The cooling circuit is either a closed circuit, in which case the heated cooling agent is cooled in a heat exchanger before flowing back into the electrical machine, or an open circuit, in which case the heated cooling agent is removed from the system.
Compared to air cooling on the outer surface, the heat transfer efficiency of liquid cooling is considerably better. However, a disadvantage of liquid cooling is that cooling can only be directed at a small area.